1. Field of the Invention
The present invention relates to a method for manufacturing a ceramic green sheet. Specifically, the present invention relates to a method for manufacturing a ceramic green sheet for use in manufacture of multilayer ceramic electronic parts such as ceramic capacitors or multilayer ceramic substrates and to a method for manufacturing multilayer ceramic electronic parts by using the ceramic green sheet manufactured by the method.
2. Description of the Related Art
Multilayer ceramic capacitors, multilayer ceramic substrates and other multilayer ceramic electronic parts are generally manufactured by a method in which ceramic green sheets are laminated, compressed and heat-treated to thereby sinter the ceramic and electrodes.
A multilayer ceramic capacitor having a structure shown in FIG. 1 is taken herein as an example. This multilayer ceramic capacitor comprises a ceramic device 1, inner electrodes 2 and a pair of outer electrodes 3a and 3b. The inner electrodes 2 are arranged inside the ceramic device 1 and are alternately derived from the right and left sides of the ceramic device 1 as illustrated in the figure, and the outer electrodes 3a and 3b are arranged at both sides of the ceramic device 1 so as to electrically communicate with the inner electrodes 2. The multilayer ceramic capacitor of this type is generally manufactured by the following method:
(1) With reference to FIG. 2, an inner electrode 2 for constituting an electric capacity is initially formed on a ceramic green sheet to thereby yield an electrode-carrying sheet 11.
(2) Next, predetermined plies of the electrode-carrying sheet 11 are laminated to yield a laminate, and ceramic green sheets (outermost-layer sheets) 21 carrying no electrode are laminated and compressed on upper and lower sides of the laminate to thereby form a laminate (unfired laminate) 1a. In the resulting laminate, the inner electrodes 2 are alternately derived from the right and left sides of the ceramic device 1 (FIG. 2).
(3) The laminate 1a is then fired under predetermined conditions to thereby sinter the ceramic, a conductive paste is applied to and baked on the right and left sides of the fired laminate (ceramic device) 1 to thereby constitute outer electrodes 3a and 3b which electrically communicate with the inner electrodes 2. Thus, a multilayer ceramic capacitor as shown in FIG. 1 is obtained.
Likewise, other multilayer ceramic electronic parts such as multilayer ceramic substrates are manufactured through a process in which ceramic green sheets are laminated to form a laminate as in the multilayer ceramic capacitor mentioned above.
Ceramic green sheets for use in the manufacture of multilayer ceramic electronic parts are generally manufactured by the following process: A ceramic powder is mixed with predetermined proportions of a dispersion medium (solvent), dispersing agent, binder, plasticizer and other additives, the resulting mixture is mixed and dispersed using medium-type dispersing apparatus such as a bead mill, ball mill, attriter, paint shaker or sand mill to yield a ceramic slurry, and the ceramic slurry is applied to a support (e.g., a carrier film) by, for example, a doctor blade process to yield a sheet having a predetermined thickness and is dried.
As the support, a poly(ethylene terephthalate) film containing an inorganic or organic powder having a particle size of several micrometers as a filler is generally used.
Demands have also been made on miniaturized and high-performance multilayer ceramic electronic parts such as multilayer ceramic capacitors as in the other electronic devices. To this end, ceramic green sheets for use in the manufacturing process of multilayer ceramic electronic parts must be very thin and must have a thickness of, for example, less than or equal to about 3 xcexcm.
However, the support (e.g., a carrier film) containing a filler having a particle size of several micrometers has prominent projections due to the filler, and the resulting ceramic green sheet has, for example, depressions about 0.3 to 2 xcexcm deep or through holes in the thickness of some parts of the sheets. The term xe2x80x9cdepressionsxe2x80x9d as used herein means depressions or blind holes that do not penetrate the sheet.
If such ceramic green sheets having depressions, through holes and other defects are used in the manufacture of multilayer ceramic electronic parts such as multilayer ceramic capacitors and multilayer ceramic substrates, they cause failures such as short-circuits and decreased withstand voltages.
In the manufacturing process of electronic parts, the resulting ceramic green sheet must be peeled from the support, and the support generally has a releasing layer including a silicone-based material on its top surface. If the top surface of the support is smoothened and the support has a releasing layer composed of a silicone-based material on its top surface (e.g., when the support is a carrier film and is wound up), slidability between two plies of the carrier film is deteriorated, and the two plies of the carrier film are adhered with each other, seriously compromising the manufacturing process of the carrier film itself or the manufacturing process of a ceramic green sheet using the carrier film.
As a possible solution to these problems, Japanese Unexamined Patent Application Publication No. 10-229027 proposes a method in which the carrier film used has a roughened undersurface (an opposite or backside surface to the top surface) and having improved slidability. However, when a ceramic green sheet is manufactured by this method, projections caused by the filler on the roughed undersurface are pressed to a top surface of the wound ceramic green sheet to thereby cause tears, through holes, projections and other defects of the ceramic green sheet.
If ceramic green sheets having these defects are used for the manufacture of ceramic electronic parts such as multilayer ceramic capacitors and multilayer ceramic substrates, they cause failures such as short circuits and decreased withstand voltages.
Accordingly, it is an object of the present invention to provide a method for manufacturing a multilayer ceramic green sheet that has a small thickness, includes no defects such as tears, through holes and projections, and has a high reliability and to provide a method for manufacturing multilayer ceramic electronic parts using a ceramic green sheet manufactured by the aforementioned method.
Specifically, the present invention provides, in an aspect, a method for manufacturing a ceramic green sheet for multilayer ceramic electronic parts by applying a ceramic slurry to a support, which method includes the steps of preparing the support, which support carrying a releasing layer on its top surface and has such smoothness that at least a region of the top surface of the support to be coated with the ceramic slurry has substantially no projections having a height of equal to or more than about 1 xcexcm, and applying a ceramic slurry to the releasing layer of the support, which ceramic slurry contains a ceramic powder dispersed in a medium.
By this configuration, a ceramic green sheet having a small thickness (e.g., a thickness of from about 0.3 to 3 xcexcm), having no defects such as tears or through holes and having high reliability can be efficiently and reliably manufactured.
The phrase xe2x80x9cat least a region to be coated with the ceramic slurry has substantially no projections having a height of equal to or more than about 1 xcexcmxe2x80x9d as used herein is not intended exclude cases where the region includes non-stationary or very local projections caused by contamination or variation in manufacturing conditions but means cases where a principle part of the support itself has no projections having a height of equal to or more than about 1 xcexcm.
The term xe2x80x9csupportxe2x80x9d as used herein means and includes carrier films that can be wound up (e.g., films of poly(ethylene terephthalate) (PET) and poly(ethylene-2,6-naphthalene dicarboxylate) (PEN)), as well as sheets, films, metallic belts, rigid plates and other articles that can be cut to a predetermined size and can be laminated, and there is no specific limitation in concrete properties, materials, dimensions and other characteristics of these articles.
The term xe2x80x9cceramic slurry including a ceramic powder dispersed in a mediumxe2x80x9d means not only a composition containing a ceramic powder dispersed in a medium (a dispersion medium or solvent) but also a composition further including a dispersing agent, binder, plasticizer, antistatic agent and other various additives in addition to the ceramic powder and medium. The ceramic slurry may comprise various substances in optional proportions according to necessity.
In the aforementioned manufacturing method, the support preferably has such smoothness that at least a region of an undersurface of the support has substantially no projections each having a height of equal to or more than about 1 xcexcm, in which no ceramic slurry is applied to the undersurface (hereinafter simply referred to as xe2x80x9cundersurfacexe2x80x9d) and the region of the undersurface corresponds to the region of the top surface to be coated with the ceramic slurry.
This configuration can prevent damage of the ceramic green sheet in contact with the undersurface when the ceramic green sheet and the support are wound up and can further reliably provide a ceramic green sheet having substantially no defects.
Preferably, the resulting ceramic green sheet obtained by the manufacturing method has a thickness of from about 0.3 to 3 xcexcm.
If the resulting ceramic green sheet is thin, conventional manufacturing methods may cause defects of the thin ceramic green sheet such as tears or through holes during manufacturing process, but the invented method can reliably manufacture a very thin ceramic green sheet having a thickness of from about 0.3 to 3 xcexcm and being substantially free from such defects.
The top surface of the support to be coated with the ceramic slurry preferably has a coefficient of static friction and a coefficient of kinetic friction each of less than or equal to about 0.45.
This configuration can efficiently wind up and transport the support to further effectively exhibit advantages of the present invention. Specifically, if the top surface of the support to be coated with the ceramic slurry has a coefficient of static friction or a coefficient of kinetic friction exceeding about 0.45, the resulting support cannot be significantly wound up after the formation of a releasing layer, and other problems may occur in the manufacturing process of the support. However, the use of a support having a coefficient of static friction and a coefficient of kinetic friction each less than or equal to about 0.45 can prevent these problems.
The surface free energy of the top surface of the substrate to be coated with the ceramic slurry is not specifically limited and is preferably less than or equal to about 55 mJ/m2.
This configuration can smooth the way to peel off the resulting ceramic sheet from the top surface of the support to thereby further effectively exhibit advantages of the present invention. If the surface free energy exceeds about 55 mJ/m2, the ceramic green sheet may become resistant to being peeled off the support, and it may take more time to peel off the ceramic green sheet without damage and thereby deteriorate production efficiency.
The coefficients of friction as indicated in the present invention are measured by the method pursuant to Japanese Industrial Standards (JIS) K-7125.
The releasing layer for use in the present invention acts to control adhesion between the ceramic green sheet and the support. The formation of the releasing layer can reduce the force (peel force) to peel off the ceramic green sheet from the support to thereby smoothly peel off the ceramic green sheet. Additionally, the releasing layer acts to prevent adhesion of two plies of a support having a highly smooth surface and to decrease coefficients of friction of the support.
The support may further have an additional intermediate layer between the releasing layer and the support base. In this case, however, the additional intermediate layer must not cause the formation of productions having a height of equal to or more than about 1 xcexcm on the top surface of the support.
In another aspect, the present invention provides a method for manufacturing multilayer ceramic electronic parts, which method includes the steps of laminating, cutting and firing plural plies of a combination of a ceramic green sheet with a base metal inner electrode to yield a sintered compact, which ceramic green sheet is manufactured by the aforementioned method, and forming an outer electrode on the sintered compact.
By this configuration, multilayer electronic parts having low short-circuit rates and other desired characteristics, high quality and high reliability and including no inner defects can be obtained.